The input stage of an operational amplifier is typically a differential transconductance cell that produces an output current in proportion to the applied input differential voltage: IOUT=gm(VP−VN) where gm is referred to as transconductance, and VP and VN are referred to as the cell's voltage inputs. In a transconductance cell, the output current relationship holds for a given range of input common mode voltages defined as VICM=(VP+VN)/2. Outside this input common mode voltage range, the transconductance of the cell becomes non-linear or non-constant. The input common mode voltage range of a transconductance cell should be as large as possible since it allows the most efficient use of the power supplies.
Another limitation of a differential transconductance cell is the maximum output current it can supply: IOUT MAX. This maximum output current usually dictates the slew rate of the operational amplifier. High slew rate is generally desired in most operational amplifier applications.
FIG. 1 shows an example of a prior art, a folded cascode transconductance cell. This transconductance cell is widely known in industry and is often used as an operational amplifier input stage. Input voltage signals VP and VN are applied to the bases of differential pair PNP transistors Q101 and Q102 and the output current is measured at the collectors of transistors Q103 and Q104, nodes IOP and ION. Q100 is a current source which provides fixed current to the emitters of transistors Q101 and Q102.
The folded cascode transconductance cell has a very wide input common mode range. The input common mode voltage can go beyond one the power rails (VEE in FIG. 1) and up to VRT+VQ101BE+VQ100CEsat from the other power rail (VCC in FIG. 1). VRT refers to voltage across resistor RT and VBE refers to the base-emitter voltage of a bipolar transistor. The maximum output current of this input stage is limited by the tail current source, Q100, of the PNP differential pair Q101 and Q102. Therefore, IOUT MAX=ICQ100.
FIG. 2 shows an example of another prior art, the X-Bridge transconductor. Q201/Q203/Q205 and Q207 form two unity gain voltage buffers for the positive input voltage VP and Q202/Q204/Q206 and Q208 form another two unity gain voltage buffers for the negative input voltage VN. These buffers apply the differential input voltage (VP−VN) across resistors RX which in turn produce the output current
      I    OUT    =            (                        V          P                -                  V          N                    )              R      X      The maximum output current of the X-bridge transconductor is limit by the base current of transistors Q205 and Q206 along with the quiescent current of current source transistors Q200a and Q200b. The theoretical maximum output current for this cell is BETA*ICQ200, where BETA refers to the current gain of a Bipolar transistor.
Compared to the folded cascode transconductance cell, the X-bridge transconductor has a relatively narrow input common mode range. Like the folded cascode cell, the common mode input voltage can go beyond one of the power rails (VEE in FIG. 2), but only up to VRT+2*VBE+VCEsat from the other (VCC in FIG. 2). Thus, the input common mode voltage range is smaller than the one for the folded cascode by one VBE.